1. Field of the Invention
The present invention relates to an optical fibre cable comprising a filling material and to the composition of said filling material.
2. Related Art
The task of the filling material in an optical fibre cable is to protect the fibres throughout their lifetime. To carry out this function, the filling material is generally introduced into the structures containing the optical fibres in such a way that the optical fibres are embedded in it.
The composition of known fillers typically comprises a base oil, for example of the silicone, mineral (naphthenic or paraffin) or synthetic type. This base oil typically requires the addition of a "viscosity improver" such as an elastomeric polymer with a low glass transition point, which by imparting viscosity to the solution improves its performance as a filling material. The filler composition may also contain an antioxidant and a thickening/thixotropic agent, consisting of more or less silanized pyrogenic silica.
There are various known types of cable which have suitable housings for the optical fibres, filled with a filling material.
For example, the patent EP 632301 describes an optical fibre cable comprising, in a suitable housing, the optical fibres embedded in a composition capable of protecting the fibres from phenomena of attenuation, in which the said composition comprises a hydrocarbon compound, a catalyst and a thixotropic thickener such as silica.
Another example of a filler composition is disclosed in patent application EP 811864, which describes a composition for fillers comprising a mineral oil or a polyalphaolefin, a block copolymer, an anti-oxidant and if necessary a stabilizer of the "metal deactivator" type.
Other examples of optical cables with known fillers are disclosed in U.S. Pat. Nos. 5,285,513 and 5,187,763 and in the patent EP 541007.
Optical fibre cables, when installed underground, for example in leads or inside underground conduits, are subject to damage affecting their coatings, caused by mechanical abuse of various kinds, for example accidental impact by sharp tools such as spades or pickaxes, which have a cutting action on the cable. The said conditions may lead to the ingress of water, with the worst possibility that it will run longitudinally inside the cable and consequently in the proximity of the optical fibres. One of the tasks of the filler is therefore to limit or impede this passage of water inside the cable structure. Less disastrous faults, such as breaks in the outer sheaths only, may also lead to permeation of water in the liquid or vapor state, with consequent harmful effects on the optical fibres, and the development of a number of problems (for example, the delamination of the layer of plastic coating of the optical fibres), the final outcome of which is manifested in optical attenuation of the transmitted signals.